Not Applicable.
Not Applicable.
The invention generally relates to mechanical pumps and more particularly to a pump that can be used to pump a fluid that may be a liquid, a gas, or a combination of both, and is particularly useful for pumping cryogenic fluids.
Cryogenic fluids, such as liquified hydrogen, oxygen, nitrogen, argon or liquified air, and liquified hydrocarbons, such as liquified methane, butane, propane or natural gas, are typically stored and transported in pressurized containers. The containers are typically well-insulated and refrigerated to very low temperatures. Pumps are used to transfer such fluids between containers or from one container to a point of use. While many types of pumps have been designed for these uses, mechanical pumps of the reciprocating type have been preferred for many applications.
Reciprocating pumps generally are required to have a net positive suction head (NPSH), i.e., a suction head above zero, to prevent the loss of prime of the pump and/or to prevent or reduce any tendency to cavitation within the pump. NPSH is the required additional pressure above the saturation vapor pressure of a liquid at any given temperature. Cavitation is the formation of vapor-filled cavities within the liquid caused in areas of the pump where the pressure of the moving liquid drops below the saturation vapor pressure. During compression the collapsing cavities may cause shock, vibration, noise, and erosion of metal surfaces, all of which can damage the pump.
The cryogenic pump disclosed in U.S. Pat. No. 5,188,519 (Splugis) includes a cylinder having a liquid inlet and a liquid outlet, and a piston reciprocally movable within the cylinder and generally intermediate the liquid inlet and the liquid outlet. The piston has a liquid flow conduit therethrough generally co-axial with the cylinder, the liquid flow conduit having an inlet end in liquid communication with the cylinder liquid inlet and an outlet end in liquid communication with the cylinder liquid outlet. A piston rod is attached to the piston for reciprocally moving the piston within the cylinder in a direction toward the cylinder liquid outlet. A valve operatively associated with and intermediate the piston rod and the piston liquid flow conduit inlet end alternately opens and closes the inlet to liquid flow, the valve being closed when the piston rod and piston are moved in the direction toward the cylinder liquid outlet and being open when the piston rod and piston are moved in the reciprocal direction.
The reciprocating cryogenic pump disclosed in U.S. Pat. No. 4,239,460 (Golz) is designed to operate with a very low NPSH. This pump uses a reciprocating piston which divides a cylindrical housing into a low pressure chamber and a smaller high pressure chamber. A gas inlet port extends through the side of the housing for channeling liquified gas into the low pressure chamber. A fixed piston extends from an outlet end of the housing into the high pressure chamber. The fixed piston slides within a cylindrical skirt carried by the reciprocating piston. Pressurized liquified gas is supplied to an outlet through a passageway within the fixed piston. One-way valves control the flow of liquified gas though the inlet, the several chambers and the outlet. During operation, the inlet fluid is compressed in the low pressure chamber in an effort to condense any gas that may come into the pump so that the resulting liquid can be forced into the high pressure chamber. If there is insufficient gas to be compressed, holes in the low pressure chamber allow excess liquid to return to a storage tank so that this chamber remains at relatively low pressure.
U.S. Pat. No. 4,447,195 (Schuck) and U.S. Pat. No. 4,559,786 (Schuck) disclose a two-stage pump, which essentially is made of two separate pumps connected by piping, having two chambers. Pumped fluid must pass through both chambers, having no intermediate way of returning to a storage tank. If the pumping results in unacceptably high pressure between the two stages of this two-stage pump, the excess pressure is vented by a relief valve.
U.S. Pat. No. 4,639,197 (Tomare, et al.) discloses a pump for cryogenic fluids that has two pistons connected by a common rod. The first piston is slightly larger in diameter than the second piston such that the second compression chamber is slightly small in volume than the first compression chamber. If there is excess liquid in the first stage of this two-stage pump, the excess liquid passes back to suction around the unsealed perimeter of the first stage piston.
U.S. Pat. No. 5,575,626 (Brown, et al.) discloses a two-stage cryogenic pump similar to the pump disclosed in U.S. Pat. No. 4,239,460 (Golz). The major difference is that this pump has an added feature, the capability to draw liquid from the bottom of a container, rather than being mounted external to a container.
U.S. Pat. No. 5,884,488 (Graham, et al.) discloses a single-stage pump intended to pump liquid only (not a two-stage pump designed for pumping two-phase fluids). Although this pump has two chambers, the second chamber within the pump is not intended to be a compression chamber. The volume of the second chamber is very large, such that the compression ratio is extremely slight. One embodiment of the pump has a first chamber and a second chamber communicating with the first chamber, a third chamber communicating with the second chamber, and a reciprocating piston separating the first, second, and third chambers from one another, and for drawing and compressing gas and liquid in any one of the chambers.
U.S. Pat. No. 5,511,955 (Brown, et al.) discloses a cryogenic pump which includes a reciprocating piston positioned in a first cylindrical housing for dividing the interior of the housing into a supercharger chamber and an evacuation chamber on opposite sides of the piston. At least one supercharger chamber inlet port extends through the cylindrical housing directly behind the reciprocating piston for channeling liquified gas from a liquified gas inlet into the supercharger chamber. A fixed piston is mounted in the housing and extends into the evacuation chamber. The fixed piston engages a skirt carried by the moveable piston to form a high pressure chamber between the movable and fixed pistons. A liquified gas outlet extends through the fixed piston from the high pressure chamber to the ultimate outlet.
There are various problems with the prior art pumps used to pump cryogenic liquids. For example, the prior art two-stage pumps do not necessarily allow all of the fluid pumped by the first stage to carry on through the pump to the discharge, the extra fluid having to be returned to a tank. Also, some of the prior art pumps require either oversized motors to allow for extra power required during the compression stroke or large flywheels to store energy during the suction stroke.
It is desired to have a two-stage pump which operates as a single-stage pump when proper fluid conditions exist at the inlet, such that all of the fluid pumped passes through to the discharge.
It is further desired to have a two-stage pump within a single housing whereby the size and cost of the pump are relatively less than the size and cost of prior art pumps.
It is still further desired to have a two-stage pump that does not require a flywheel or oversized motor to store energy, but rather stores energy as pressure inside the pump during the suction stroke.
It also is desired to have an improved reciprocating pump for pumping cryogenic fluids which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.
The present invention is a reciprocating pump for pumping at least one fluid that may be a liquid, a gas, or a combination of both. A first embodiment of the pump includes a housing, a piston slideably mounted within the housing for a reciprocating movement, a shaft connected to the piston and adapted for reciprocating movement concurrently with the piston, an inlet valve, a discharge valve, and an interstage valve means. The housing has a longitudinal axis, at least one inner wall, a first end, a second end opposite the first end, an inlet adjacent the first end, a discharge between the second end and the inlet, and an open interior between the at least one inner wall and the first and second ends. The piston is slideably mounted within the housing for a reciprocating movement substantially parallel with the longitudinal axis. The piston has a first cross-sectional area, a front end facing the first end, and a rear end opposite the front end, and divides the open interior into a first chamber having a first volume adjacent the inlet and a second chamber having a second volume adjacent the discharge. The first and second volumes vary inversely with the reciprocating movement of the piston. The first chamber is in controllable fluid communication with the inlet, the second chamber is in controllable fluid communication with the discharge, and the first and second chambers are in controllable fluid communication. The shaft, which has a forward end connected to the rear end of the piston and a rearward end opposite the forward end, is adapted for the reciprocating movement concurrently with the piston, and at least part of the shaft is disposed in the second chamber. The inlet valve is in communication with the inlet and is adapted to control the fluid flowing through the inlet to the first chamber. The discharge valve is in communication with the discharge and is adapted to control the fluid flowing from the second chamber through the discharge. The interstage valve means is in communication with the first chamber and the second chamber, and is adapted to control the fluid flowing from the first chamber to the second chamber. The interstage valve means is closed during a suction stroke and is open during a compression stroke, the suction stroke and the compression stroke occurring in an alternating manner during the reciprocating movement of the piston.
Various types of fluid may be pumped by the reciprocating pump, including but not limited to cryogenic fluids. In one variation, at least a portion of the fluid is a single phase fluid. In another variation, at least a portion of the fluid is a two-phase fluid.
In a preferred embodiment, the piston is moveable relative to the housing and the housing has a fixed position. In another embodiment, the piston has a fixed position and the housing is moveable relative to the piston.
In the preferred embodiment, the part of the shaft disposed in the second chamber has a second cross-sectional area substantially equal to about one-half the first cross-sectional area.
In another embodiment, the pump includes sealing means adapted to provide at least one seal between the inner wall of the housing and an outer surface of the piston in an alternating manner during the reciprocating movement. Preferably, the seal is provided during the suction stroke. The preferred sealing means includes at least one piston ring mounted peripherally on the piston.
In the preferred embodiment, at least part of the interstage valve means is mounted on the piston. However, other variations are possible. In one variation, the interstage valve means includes an inlet port in the inner wall adjacent the first chamber, a discharge port in the inner wall adjacent the second chamber, and transfer means adapted to transfer at least part of the fluid from the inlet port to the discharge port, the inlet port being in fluid communication with the first chamber and the discharge port being in fluid communication with the second chamber.
In addition, there are other embodiments of the invention, a reciprocating pump for pumping at least one fluid. One such embodiment includes a housing, a piston slideably mounted within the housing for a reciprocating movement, a shaft connected to the piston and adapted for a reciprocating movement concurrently with the piston, means for controlling the fluid through the inlet to the first chamber, means for controlling the fluid flowing from the second chamber through the discharge, and control means for controlling the fluid flowing from the first chamber to the second chamber, wherein the control means is closed during a suction stroke and is open during a compression stroke, the suction stroke and the compression stroke occurring in an alternating manner during the reciprocating movement of the piston. In this embodiment, the housing, the piston, and the shaft are all substantially the same as or similar to the housing, the piston, and the shaft of the first embodiment described above.
This alternate embodiment also may be used to pump various types of fluids, including but not limited to cryogenic fluids. In one variation of this embodiment, the part of the shaft disposed in the second chamber has a second cross-sectional area substantially equal to about one-half of the first cross-sectional area.